Bow thruster battery

In September’s magazine Nigel Calder answers a reader’s query about installing a bow thruster battery. Here is the full text of the question and reply

>> Dear YM,
>>
>> I wish to fit a Bow Thruster which is rated at 575Ah to my sailing yacht.
>> The engine is normally running at low revs for perhaps 30 to 40 minutes at
>> the start and at the end of each trip. The Bow Thruster will be used,
>> probably, for up to two minutes for maneuvering at the start and end of each
>> trip. The Bow Thruster can be run for a maximum of three minutes in any
>> hour.
>>
>> The battery will be positioned next to the Bow Thruster which therefore
>> requires approximately an eleven meter cable run for the charging cable –
>> from the engine alternator (80Ah rated) to Bow Thruster battery.
>>
>> I am no electrician; but refering to Mr Nigel Calder’s excellent manual I
>> calculate that the discharge from any battery from 3 minutes use would be
>> 29Ahs. For a 80Ah battery to replenish this discharge in my normal running
>> times – if in practice it actually can – will require a cable size as fat as
>> my index finger.
>>
>> Please advise the size (capacity) and type of 12V battery you would
>> recommend. (I would prefer AGM from choice). Also please advise cable size
>> for an eleven meter cable run to charge
>> this recommended battery.
>>
>> Many thanks,
>>
>> Bryan o’Brien

Nigel Calder replies:
As you note, the power drain of a bow thruster is huge. This raises a couple of issues. First, the thruster will suck the voltage down on any but the largest battery bank. Second, because the voltage is down, if the engine is running (which it almost certainly will be) the engine-driven alternator is likely to go to full output. What this means is you want the largest battery you can handle up forward to minimize the voltage drop while the bow thruster is in operation, and, ideally, charging cables to the battery that will handle full alternator output with as little voltage drop as possible.

Putting some numbers to this, at 12 volts the bow thruster drain may be anything up to 500 amps (depending on size). I think it highly unlikely you will ever use a thruster for 3 minutes in any one manoevering session – typically, we are looking at seconds rather than minutes – but lets allow for 3 minutes with a 500 amp drain. That’s 25 amp hours. For this kind of high discharge application, I’d like to see the battery’s amp-hour rating at ten times the discharge – i.e. 250 amp-hours. That’s a big battery! So let’s scale things back a bit and assume we are not likely to use the thruster for more than a minute or two, and accordingly set a minimum battery target of 100 amp-hours. Somewher between 100 and 250 amp-hours, depending on available space and weight issues, will get the job done.

So far as recharging is concerned, if we assume the alternator is maxed out (80 amps in this case) while the thruster is in operation, and try to size the cables for a minimum voltage drop (ideally 3%) the cables will ,as you say, be extremely large (I am in the middle of a trans-Atlantic crossing at the moment without those tables in front of me so cannot give the exact size). However, the minute you stop using the thruster, the battery voltage will bounce back, the alternator output will go down, and the voltage drop will decrease. So in practice it is acceptable to base the charging cable size on something well under full alternator output – let’s say 40 amps, with a 3% volt drop. If you’ve got a copy of my ‘Boatowner’s Mechanical and Electrical Manual’ three’s a table there where you can look this up. If possible, err on the large size to minimize voltage drop because whatever voltage drop you get in these cables will mean this battery is seeing a lower charging voltage than the other batteries on the boat, and as such the thruster battery will tend to be perennially undercharged, which will tend to shorten it’s life.